The present invention relates to an improved color monitor, and in particular to a color monitor for displaying images and/or texts in a plurality of colors.
As known, the use of color monitors connected to numeric devices, such as computers, for the display of images or texts in a plurality of colors is currently becoming widespread.
In particular, color monitors are currently commercially available which are constituted by a single cathode-ray tube which is capable of generating a single electron beam with different energy levels so as to excite in a differentiated manner a screen coated with electroluminescent material which therefore emits polychromatic light.
A color monitor of this type is described for example in the Italian patent application 19217A/87, filed on Jan. 30, 1987 in the name of the same Applicant. In particular, said known monitor comprises a circuit for controlling the cathode-ray tube which receives in input electric color signals which are appropriately encoded and represent the primary colors to be visualized on the screen, and modifies the level of energization of the electron beam at the end of each complete scan of the screen so as to separately display in succession the primary colors which constitute the images to be shown.
By virtue of the phenomenon of retention on a part of the retina and to the persistence of the light emission of the electroluminescent material, even if the primary colors are emitted at different times they are seen by the human eye as if they were emitted simultaneously, thus allowing the visualization of many or all colors using only two or three primary colors.
In this kind of monitor, amplifier means are thus provided which are capable of adapting the anode current or anode voltage according to the color to be displayed in each instance, thus modifying the energy of the generated electron beam according to the color to be visualized in that instant.
At the end of each scan of the screen, therefore, the anode voltage must be increased or decreased to visualize the subsequent color. This variation must be performed during the electron beam flyback time in order to correctly display the colors of the image being shown. Whereas the anode voltage increase does not entail substantial problems, the adaptation of the energy of the electron beam in changing from a higher energy level to a lower one is instead more troublesome. The anode (connected to the high voltage) is in fact formed by an aluminized layer or by a coating of another conducting material inside the glass bulb which forms the electron gun and is externally surrounded by a layer of graphite connected to the ground. The anode and the graphite therefore form the two plates of a capacitor, the dielectric whereof is constituted by the glass of the bulb. The capacitor must be discharged in passing from a higher voltage level to a lower one within the flyback time of the electron beam.
The discharge of the capacitor therefore constitutes a problem for the correct operation of the monitor which has been dealt with in various manners. For example, it is possible to provide a resistor which is connected in parallel to the capacitor to allow its discharge. Since this solution requires a longer time for discharge, the color change command is issued with a delay with respect to synchronization; if this delay is chosen equal to at least 180.degree., in practice it entails an advance of the color change instant. In practice the discharge of the capacitor can start immediately after the end of the scan of the screen, without waiting for the synchronization signal, thus allowing the correct operation of the monitor.
Though this solution is very advantageous, since it solves the above described problem with very simple and economical means, it is however susceptible to improvement, in particular in order to increase the capacitor discharge speed.